Foundation personnel wishing to take advantage of SCP-XXXXc's anomalous properties are to approach the 'attendant' and start a short conversation during which they shall speak the codewords "lavender", "wrinkles", and "polyester". Upon hearing all three codewords, the 'attendant' shall issue a packet of SCP-XXXXc-1 to the Foundation patron.

This is needlessly complex and a containment breach would be nigh freaking guaranteed with this system. It'd make more sense to say that the deep-cover agent ships SCP-XXXXc-1 to a Foundation Site whenever it's restocked; personnel wishing to test must yadda yadda yadda. We're not the Unusual Incidents Unit. Such precautions would also negate the need for the third paragraph of the procedures. They'd probably hang little "out of order" signs on the anomalous washers as well.

two Foundation agents are to keep watch over SCP-XXXXc from a car on the other side of the street to monitor SCP-XXXXc-2

Minor nitpick, but "from a car on the other side of the street" sounds a bit casual.

SCP-XXXXc is a laundromat located in downtown ██████████, Pennsylvania, USA.

This brings me to one of my main points of concern. SCP-XXXXc is not the the laundromat, it's the soap packets. Objects are only given an SCP-[number]-X classification if they are, in some way, anomalous. The laundromat is not in and of itself anomalous (except for Incident XXXXc-1, which I'll get to).

any of the 14 washing machines (designated SCP-XXXXc-A through SCP-XXXXc-N)

The only anomalous washing machines are SCP-XXXXc-A, -B, -C, -G, and -L. The others exhibit no anomalous properties and their classification as such clutters the article.

It appears to be a perfectly normal laundromat, aside from its windows which have been painted over black from the inside.

Poor clinical tone here; I'd recommend snipping it and moving the black-window thing to somewhere else in the article.

SCP-XXXXc-1 is a liquid laundry soap initially found within SCP-XXXXc when it came under Foundation control.

Two things. First, XXXXc-1 should be reclassified XXXXc, since it's the main thing of the article. Second, this is one of the few times I'd really like to read an origin story of how the Foundation discovered this thing.

Clothes washed in this manner continue to exhibit anomalous properties for a period of seven (7) days, after which the effects dissipate within a matter of hours.

After the comma, this reads poorly. Personally, I'd write "… seven (7) days; after this time has elapsed, anomalous effects persist for one (1) to five (5) [or whatever numbers] hours before disappearing."

in ordinary non-anomalous washers

Redundancy. Ordinary = non-anomalous.

SCP-XXXXc-2 appears to be a Latino male

This might just be a thing that sticks out to me in particular, but the use of "appears to be" is pretty jarring in articles, unless it actually appears to be one thing but is actually another if observed in a different way.

And speaking of XXXXc-2, what's up with him anyway? Like, did the Foundation ever try to detain this guy? Or talk to him? The addendum suggests that he can be "brought in for questioning" as if he's actually a normal dude, but nothing else is mentioned in the article proper. It seems like a little bit of a missed opportunity.

SCP-XXXXc Archived Testing Logs

Not quite critique, but just a little aside. Shit like this is absolutely lovely in my opinion. The reason SCP-261, SCP-914, SCP-1147, and SCP-294 are some of my favorite articles on the site basically boils down to "cool testing logs with something that works both predictably yet unpredictably at the same time." That said, tread with caution. Good articles with good test logs sometimes get downvoted simply because "Things what you put other things in and they come out improved" is on The Big List Of Overdone SCP Cliches.

Results: [DATA EXPUNGED]

This is a poor expungement. Especially because it's basically "the bras were invisible ha-ha crude humor." Either make it dry, make it actually funny, give it dry humor, or take it out.

Addendum: I petition Site Command that Dr. Bright no longer be allowed to offer me gifts, nor go with me on morning jogs. - Dr. Rights

Mentioning author avatars such as Bright & Rights was indeed something done in the past in the 001-999 articles, but now it's considered really poor form and something that you shouldn't do. Basically, lolFoundation is bad.

Further testing with striped and patterned clothing revealed that each discrete color was randomized, not simply the entire garment.

What?

some stuff I forgot to mention

It's worth noting that each washing machine has a distinct and repeatable effect. It's something I didn't even figure out before the last test, so you should probably make it clearer in the article. That, or you could make the washing machine's effects completely random, or affected by something changeable such as the type of clothing you put in. That might lead to a more interesting test log, but that might be my penchant for long test logs (see above) speaking.

Addendum: Incident Report XXXXc-1

Personally, I think this entire addendum is bland and doesn't add anything to the article. It feels tacked on, as weird as that may sound (since addenda are literally just pieces of tacked-on information). I think it would make more sense to just say that the laundromat is completely sealed when XXXXc-2 is in there.

Now, with all that critique out of the way: if you gave this article a bit of a tune-up, I'd upvote the hell out of it. Is it "thing what you put other things in and they come out improved?" Yes. Does this make it bad? Hell no, partially because it does it in an interesting way and partially because it does it fairly well.